Threshold Shear Strain Research Articles

Errata for “Threshold Shear Strain for Cyclic Pore-Water Pressure in Cohesive Soils” by Chu-Chung Hsu and Mladen Vucetic

Errata for “Threshold Shear Strain for Cyclic Pore-Water Pressure in Cohesive Soils” by Chu-Chung Hsu and Mladen Vucetic

Threshold Shear Strain for Cyclic Pore-Water Pressure in Cohesive Soils

Threshold shear strain for cyclic pore-water pressure, γt , is a fundamental property of fully saturated soils subjected to undrained cyclic loading. At cyclic shear strain amplitude, γc , larger than γt residual cyclic pore-water pressure changes rapidly with the number of cycles, N , while at γc < γt such changes are negligible even at large N . To augment limited experimental data base of γt in cohesive soils, five values of γt for two elastic silts and a clay were determined in five special cyclic Norwegian Geotechnical Institute (NGI)-type direct simple shear (NGI-DSS), constant volume equivalent undrained tests. Threshold γt was also tested on one sand, with the results comparing favorably to published data. The test results confirm that γt in cohesive soils is larger than in cohesionless soils and that it generally increases with the soil’s plasticity index (PI). For the silts and clay having PI=14–30, γt =0.024–0.06% was obtained. Limited data suggest that γt in plastic silts and clays practically...

Mechanical Shear Can Accelerate the Gelation of Actin Filament Networks

Rearrangements of the filamentous actin cytoskeleton at the leading edge of motile cells occur under large mechanical stresses. Contrary to conventional wisdom, we show that mechanical deformations applied during gelation can accelerate the rate of gelation and produce F-actin networks that are stiffer and mechanically more resilient than those polymerized under low or high shear deformations. Above a threshold shear strain amplitude, F-actin networks collapse and become soft and liquidlike. This effect of shear-induced strengthening of polymerizing networks depends on the state of hydrolysis of the actin-bound adenosine triphosphate.

Volumetric Threshold Shear Strain for Cyclic Settlement

The volumetric cyclic threshold shear strain for cyclic settlement, γtv, is evaluated for seven different sands and clays at different degrees of saturation, S, from the results of 11 Norwegian Geo...

Numerical analysis of a multipropped excavation in stiff clay

This paper presents the procedures and the results of numerical analyses of a multipropped deep excavation at Lion Yard, Cambridge, using the nonlinear Brick model. The computed results are compared with the comprehensive case record. The observed small deflections and bending moments of the wall, low prop forces, and relatively small ground movements during the main excavation have been taken into account. Shear strains which developed around the site during the main excavation were generally less than 0.3%. Significant reduction of lateral stress in the ground during wall installation and the highly nonlinear stress-strain characteristics of the Gault Clay are the chief reasons for the observed unusual behaviour. The Gault Clay exhibits first yield at a threshold shear strain of about 0.001%, beyond which the stiffness deteriorates significantly from an initial very high value. This high stiffness at very small strains may be due to cementation bonding between soil particles, as a result of the presence of calcium carbonate. Simple drainage assumptions for the stiff fissured clay on both sides of the diaphragm wall appear to be inadequate for design analyses.Key words: numerical analysis, multipropped excavation, Gault Clay, nonlinear brick model, small strain stiffness.

Determination of Threshold Ground Acceleration for Soil Liquefaction Using the Strain and Stress Approaches

The evaluation of soil liquefaction using the stress approach utilizes the relative density and initial confining stress available at the layer subjected to liquefaction. The shear strain approach makes use of the shear modulus which is a more fundamental property of materials. The threshold ground acceleration values for forty layers at ten selected sites were evaluated using the stress and strain approaches. For the stress approach, the method proposed by Shibata and Teparaksa (1988) based on the concept of critical cone tip resistance was selected. For the strain approach, the method proposed by Dobry and others (1981) was employed. The threshold ground acceleration values computed using the stress approach were found to be two to four times higher than those computed using the strain approach. Several scenarios were tested in order to incorporate the effect of variations in the threshold shear strain and the ratio of (G/Gmax)t. The results of the stress and the strain methods are based on site response analyses performed using the program SHAKE91 and the input ground motions of the 1988 Saguenay earthquake.

Cyclic Simple Shear Apparatus for Low-Strain Soil Tests

A testing system has been developed to study the behavior of saturated sand under low-level cyclic shearing strains. The system has been used to determine threshold shear strain levels for fabric destruction in sand aged for different time periods. The system includes a special soil chamber and a direct simple shear (DSS) machine. To impose very small shearing strains, the DSS machine was designed to apply and measure horizontal deformations as small as 0.0005 mm (2 × 10−5 inches). Data obtained to date support the results of previous investigators who performed triaxial tests on freshly deposited samples, indicating a threshold cyclic shear strain level of approximately 0.01 percent. At strains in excess of those levels, destruction of the sand fabric occurred, as evidenced by a reduction in shear modulus at low strain levels. Subsequent modest increases in shear modulus were observed after the specimens were allowed to recover for 24 hours and then tested again. During the recovery period, drainage valves were left open to allow for dissipation of excess pore pressures and for potential consolidation during the short aging period. The DSS system was found to work well for low strain measurements. Furthermore, since shear strains are measured directly under DSS conditions (as opposed to triaxial conditions), the DSS system shows much promise as a device for studying parameters that may influence threshold shear strain levels and fabric evolution and destruction in sands.

Cyclic Simple Shear Apparatus for Low-Strain Soil Tests

A testing system has been developed to study the behavior of saturated sand under low-level cyclic shearing strains. The system has been used to determine threshold shear strain levels for fabric destruction in sand aged for different time periods. The system includes a special soil chamber and a direct simple shear (DSS) machine. To impose very small shearing strains, the DSS machine was designed to apply and measure horizontal deformations as small as 0.0005 mm (2 × 10−5 inches). Data obtained to date support the results of previous investigators who performed triaxial tests on freshly deposited samples, indicating a threshold cyclic shear strain level of approximately 0.01 percent. At strains in excess of those levels, destruction of the sand fabric occurred, as evidenced by a reduction in shear modulus at low strain levels. Subsequent modest increases in shear modulus were observed after the specimens were allowed to recover for 24 hours and then tested again. During the recovery period, drainage valv...

The Small Strain Stiffness of a Carbonate Stiff Clay

ABSTRACT Knowledge of the small strain stiffness of the heavily overconsolidated Gault Clay, which has up to 30% calcium carbonate content, is rather limited. This has resulted in some difficulties in the analysis and design of structures constructed in the Gault. In this Technical note, the small strain stiffness of the Gault Clay is examined based on results from triaxial tests with internal small strain measurements, published geophysical data, and values deduced from the back-analysis of full-scale field observations of an excavation at Lion Yard Cambridge, U.K. Comparisons of stiffness values have also been made between the Gault Clay and the non-carbonate heavily overconsolidated London Clay. The results of these examinations have led to the conclusion that the stiffness-strain characteristic of Gault Clay is highly non-linear and exhibits first yield at a threshold shear strain of about 10− 5, beyond which the stiffness deteriorates dramatically from an initially very high value. After modest straining the stiffness reduces to values comparable to those for London Clay. The Gault Clay behaves like a low plasticity clay at small strains but as a high plasticity clay at medium to large strains. This behaviour is probably due to the breakdown of the weakly cemented bonding caused by the calcium carbonate content.

Cyclic threshold shear strains in soils

Cyclic threshold shear strains in soils

Generalized Cyclic-Degradation-Pore-Pressure Generation Model for Clays

This paper proposes a model for the behavior of fully saturated normally consolidated and overconsolidated marine clays subjected to uniform cyclic strain-controlled and staged cyclic loading under simple shear conditions. To simulate a clay response similar to that occurring during the irregular cyclic loading of earthquakes, ocean storms, and other natural phenomena, staged cyclic testing with several levels of different uniform cyclic strains was performed. Cyclic degradation, cyclic pore-water pressure generation, and cyclic threshold shear strain were incorporated into the model. Cyclic threshold shear strain was defined as the cyclic strain amplitude below which neither significant pressure develops nor significant degradation occurs. The model was verified by staged cyclic direct simple shear testing of five marine clays. Practical applications of the model are discussed.

Cyclic Threshold Shear Strains in Soils

Based on a synthesis of published laboratory data, two types of cyclic threshold shear strain are examined and their approximate magnitudes identified for different types of soils. They are the linear cyclic threshold shear strain, γtl and the volumetric cyclic threshold shear strain, γtv, with γtv>γtl. These strains represent boundaries between fundamentally different categories of cyclic soil behavior. For cyclic strains below γtl, soil behaves essentially as a linearly elastic material. Between γtl and γtv, soil becomes markedly nonlinear but remains largely elastic because permanent changes of its microstructure still do not occur or are negligible. Above γtv, soil becomes increasingly nonlinear and inelastic, with significant permanent microstructural changes taking place under cyclic loading. That is, γtv, = the threshold separating cyclic strains that cause or do not cause significant permanent changes of soil microstructure. In practical terms, these microstructural changes are manifested in resid...

Settlement of Compacted Clay in a Cyclic Direct Simple Shear Device

Abstract The results of 36 cyclic strain-controlled direct simple shear tests on a low-plasticity compacted clay are presented and analyzed. The tests were conducted in a Marshall Silver-type device, which utilizes the Norwegian Geotechnical Institute (NGI) type of short cylinder specimen confined in a wire-reinforced rubber membrane. Three degrees of compaction achieved by the modified compaction test are encompassed: (1) below the optimum moisture content, (2) at the optimum, and (3) above the optimum. All specimens were sheared under the same vertical consolidation stress. In each test the cyclic shear strain amplitude, γc, was controlled, i.e., the cyclic tests were strain controlled. The range of γc was between 0.008 and 4.6%. The test results show a very consistent behavior of the clay at all three moisture contents. At small cyclic shear strains below γc ≅ 0.1%, the stress-strain behavior is slightly nonlinear, i.e., close to linearly elastic, and the vertical settlement is negligible. In some tests such nondestructive behavior was recorded up to γc ≅ 0.2%, showing that for the clay tested, the volumetric threshold shear strain, γtv, ranges between 0.1 and 0.2%. At γc larger than γtv the cyclic stress-strain behavior becomes nonlinear and a continuous settlement with the number of cycles, N, occurs. The results do not show a clear relation between the rate of settlement with N and the degree of the compaction and moisture content. For different moisture contents, similar settlements were obtained for given γc and N. The study also shows how to arrange the specimen setup and measuring system to eliminate the effect of the simple shear apparatus compliance. In such an arrangement very small shear strains can be applied and successfully measured.

A Pore Pressure Model for Cyclic Straining of Clay

The paper focuses on the development and modelling of residual cyclic pore water pressure, u, in clay during undrained cyclic strain-controlled loading. Data obtained from a series of NGI type cyclic simple shear tests were employed. In addition to the amplitude of cyclic shear strain, γ c , the number of cycles, N, and the overconsolidation ratio, OCR, a new component is incorporated in the conventional characterization of u. This new component is the volumetric threshold shear strain, γ tv , of clayey soils, below which for all practical purposes u does not develop. The lack of understanding of complex clay microstructure and the associated interaction between clay particles during cyclic shearing makes the modelling of u in a manner other than curve fitting rather difficult. This is especially true in regard to the development of negative u in overconsolidated clays. A model based on the systematic curve fitting of the pore pressure data expressed in terms of γ c , γ tv , N and OCR is presented. The possibilities of the incorporation of such a model into existing numerical tools for simulation of the seismic and ocean wave loading response of natural soil deposits are briefly discussed as well.

Geotechnical aspects of earthquake‐induced settlement of clay layer

Abstract It has been observed that earthquake‐induced settlement depends on the excess pore water pressure accumulated during an earthquake. In particular, in the case where a clay layer is overconsolidated, excess pore water pressure is produced and settlement occurs by dissipation of the excess pore water pressure, which is very large in comparison with the coefficient of secondary compression. Therefore, if the settlement of clay ground induced by secondary compression becomes a serious problem, careful consideration of the earthquake‐induced settlement is needed. In this article, the settlement characteristics of a clay layer induced by cyclic shear are discussed, including the effects of loading period, the threshold shear strain below which no excess pore pressure or no settlement takes place, and the relationships between uniform shear strain cycles and irregular strain‐time histories. Then a calculation procedure for estimating the earthquake‐induced settlement is developed and applied to three soil profile cases, including the clay layers in Mexico City and Osaka Bay in Japan. In the case of a soil profile in Mexico City, settlements of about 0–3 cm are estimated and these values agree reasonably with the leveling results for the Mexico City earthquake of 1985. Furthermore, it is pointed out that the settlement induced by earthquakes is considerably affected by differences in the accelerograms.

Pore-Water Pressure Buildup in Clean Sands Because of Cyclic Straining

Abstract The prediction of pore-water pressure buildup in sands caused by undrained cyclic loading is one of the key items in evaluating the potential for liquefaction of sandy sites during earthquakes. Presented herein are data indicating that, in strain-controlled tests, there is a predictable correlation between cyclic shear strain, number of cycles, and pore-water pressure buildup; this correlation is much less sensitive to factors, such as relative density and fabric than comparable results obtained from stress-controlled tests. Also, there is a threshold cyclic shear strain below which there is no sliding at the contacts between sand particles, and essentially no pore-water pressure buildup occurs. The data indicate that, for clean sands, this threshold shear strain, as well as the pore-water pressure buildup for strains slightly above the threshold, are basically independent of relative density, grain size distribution, fabric, and method of testing (triaxial and direct simple shear). However, both threshold shear strain and pore-water pressure buildup do depend on the overconsolidation ratio.

Pore Pressure Buildup in Resonant Column Tests

Resonant column tests were performed on fully saturated and dry hollow cylindrical and fully saturated solid cylindrical specimens of Monterey No. 0 sand of 60% relative density subjected to 96 kPa confining pressure. The hollow specimens were tested by torsional excitation and the solid specimens by longitudinal excitation. It was observed that the threshold shear strain amplitude below which no pore water pressure buildup occurs is approximately 1.5 x 10\u-³ for specimens which were not previously shaken. Between the approximate shear strain amplitudes of 1.5 x 10\u-³ and 8.5 x 10\u-³% there was a cyclic strain hardening effect which increased the threshold strain amplitude for specimens which were previously shaken, and caused the pore water pressure of virgin specimens subjected to cyclic strain to stabilize at a predictable level (liquefaction did not occur). All specimens, whether previously shaken or not, liquefied at the threshold strain of 1.2 x10\u-²% which was previously identified to be the shear strain below which gross particle displacement is frictionally blocked. The effect of previous shaking on damping ratios and shear moduli was also investigated.

Centrifugal Modeling of Soil Liquefaction Susceptibility

Presented are the results of an experimental research program involving centrifugal model tests on a column of sand confined by a stack of teflon coated aluminum rings with a latex membrane between the sand and the rings. Dynamic centrifugal model testing was performed in a 1 m radius Schaevitz centrifuge using a piezoelectric shaker. A column of Monterey 0 grade sand contained in a stacked-ring apparatus modelled a sand layer. The sand layer was instrumented with pore pressure transducers at different depths and accelerometers at the base and top. The model was excited at the base with sinusoidal motions of various amplitudes and the excess pore water pressures and the ground surface accelerations were monitored. The principal objectives were to study the generation and dissipation of excess pore water pressures, and to estimate the threshold peak ground surface acceleration, and the threshold shear strain necessary for the initiation of excess pore water pressure. The influence of prior strain history on the generation of pore water pressure was an additional objective of this study. The study showed that the meaured time history of excess pore pressure generation and pore water pressure distribution with depth compared favorably with that predicted by the currently available numerical procedures using dynamic test results obtained on small elements of homogeneous sand. The study also showed the significance of the initial prior strain history on pore pressure development and the accuracy of the peak threshold acceleration and shear strain estimated from dynamic tests on small elements of homogeneous sand in cyclic triaxial tests.